1. Field of the Invention
The present invention generally relates to a compact disc (“CD”) player. More particularly, the invention relates to a compressed audio disc player that spins a disc at a constant linear velocity that is keyed to the encoding rate of the compressed audio. Still more particular, the invention relates to an encoding rate-based, programmable linear velocity compressed audio disc player.
2. Background of the Invention
In recent years, audio compact discs (“CDs”) and CD players have grown in popularity. A conventional CD stores audio in a digital format on the disc. The digital data is accessed by an optical pickup mechanism in the player as the CD is rotated adjacent the optical pickup. The player includes electronics such as a motor, motor drive and control circuitry, decoder logic, and the like to spin the CD at an appropriate speed, read the data off the CD, process the data, and convert it to an analog format to be played through a speaker or headphones.
The circuitry in a conventional CD players spins the CD at a constant linear velocity (“CLV”). The data is stored on the CD in a generally spiral track with each bit on the track spaced at an equal distance from adjacent bits. Audio on a conventional audio CD has been digitized, but not compressed. The CD is spun at a rate that permits the optical pickup to extract data from the CD into a bit stream that matches the rate at which the audio data must be played through a speaker. That is, if X number of bits of digital data represents one second of live audio, then the CD is spun at a rate that permits a bit stream to be created by the optical pickup that comprises X bits per second. This X bits per second bit stream can then be piped through a speaker. The equal spacing of bits on the spiral track requires the rotation speed of the CD to decrease as the optical pickup spirals away from the center of the disc.
Because conventional audio CDs do not store data in a compressed format, a single CD typically can hold about 67 minutes of audio. It is desirable, of course, to be able to store much more audio on a CD and, in general, as much as possible. To that end, compression techniques have been implemented to compress digital audio data and store such data in a compressed format on a CD. The well known “MP-3” compression standard is becoming popular for enabling CDs to contain much more audio than a conventional CD. MP-3 compression ratios of, for example, 10:1 can be obtained to permit ten times more audio to be stored on a single MP-3 CD than on a conventional (i.e., uncompressed) CD.
Although permitting much more audio to be stored on a CD is highly advantageous, it is not problem-free. For instance, the same electronic drive and control electronics are often used in MP-3 players as were used in conventional CD players. This means that an MP-3 CD is rotated at the same constant linear velocity as was a conventional CD. However, the bit stream coming off the disc is compressed and thus represents more audio than the bit stream from a conventional audio CD. In a conventional CD player, the disc is rotated at the CLV so as to produce a 1.44 megabits per second and that 1.44 megabits represents one second of live audio. Each second, however, in the bit stream from an MP-3 CD spun at the same CLV represents more than one second of live audio (e.g., ten seconds of live audio) because the data is compressed. In general, data is being pulled off an MP-3 CD at a faster rate than it is being consumed (e.g., played through a speaker).
This problem has been addressed by adding memory and other logic into the player to be able to store the data coming off the disc into a relatively large memory buffer and playing the data from the buffer. Once the buffer becomes full, the player pauses the optical pickup mechanism to stop reading data off the disc. The player continues pulling data out of the buffer to be consumed. Once the amount of unconsumed data in the buffer falls below a threshold, the player's optical pickup is re-activated to resume reading more data off the disc to again fill up the buffer. Thus, rather than extracting data from the CD at a constant rate (i.e., the rate at which it is consumed), the data is extracted from the CD in shorter duration bursts.
Although this approach is generally satisfactory, it suffers from two deficiencies. First, the extra hardware necessary to implement this approach is significant. The memory buffer in which the compressed data is burst loaded from the CD must be significantly large which requires extra battery power (in the context of portable player), space, and adds significant cost as well. Second, during a pause, the optical mechanism is held fixed by applying a constant voltage to the spring-loaded radial voice coil actuator. The player may be bumped during the pause time period thereby moving the optical pickup relative to the CD. Thus, after a pause, when the optical mechanism is to begin reading more data off the CD, the exact location at which the mechanism left off must be located and the optical mechanism must be moved to that location. The process of moving the optical mechanism to the correct location on the CD is called a “re-seek.” The re-seek process consumes power which is not desirable, particularly for battery operated devices. Accordingly, a solution to these problems would be highly desirable.